1. Field of the Invention
The present invention concerns an ionic transducer for measuring the transit time between an emission area and a receiving area, associated with electronic processing equipment, and more particularly a transducer for measuring the volume flow of a stream of gas by using ionized particles as a reference.
The transducer of the present invention is of the transit-time anemometer type; it is designed for measuring an air flow with a view to controlling the richness of the air-fuel mixture fed into an internal combustion engine.
2. Description of the Prior Art
The principle of transit-time anemometers consists in using a cloud of ions as a moving body induced to travel at the same velocity as the gas flow, with the time it takes the cloud to pass between an emission device and a receiving device being measured. The emission device is generally a device (a point or wire) which receives a short, high-voltage impulse and therefore induces localized ionization. The cloud of ionized gas thus formed induces a signal by its influence when it passes through a receiving device.
In many cases, the application of this principle poses difficulties.
On the one hand, the time measured between emission and reception is replete with errors because the phenomenon of ionization is taking place during only part of the duration of the high-voltage impulse; the phenomena of ionization and movement of the stream of gas are superimposed.
On the other hand, the high-voltage impulse (several kilovolts) on the emission device induces a very high voltage on the receiving device by virtue of direct radiation; this is superimposed over the real induced signal and threatens to destroy the electronic system set up in order to amplify the said induced signal.
Moreover, the geometric position of the emission system in the stream of gas causes vortices and wakes, thus destabilizing the signal received and constituting an additional error factor.
Finally, it is known that the flow of air in the intake manifold of a thermal engine is pulsating rather than continuous, i.e., that over the average value of Qv.sub.moy is superimposed a variable sinusoidal value whose frequency is twice that corresponding to the motor's velocity of rotation. It is hence necessary in this case for the measurements taken by the transducer to be carried out at particular instants which correspond to the passage of the pulsating flow at the average value Qv.sub.moy. To do this, it is therefore necessary to use an electronic control device which observes these operating conditions.